The invention relates to a device for determining a quantity of consumable product present in a reservoir comprising for example at least two compartments, one where the product is in the free state and the other where it is in the captive state.
The invention relates particularly to determination of the quantity of ink remaining in such a reservoir equipping a printing device, more particularly a printing device using ink-jet technology.
A number of ways for detecting a level of ink in a reservoir of a printing device are known. Notably, association with the reservoir is known of a capacitive arrangement to which an excitation signal, generally a high-frequency alternating signal, is applied. It is known that the signal transmitted by such a capacitive arrangement varies according to the quantity of ink remaining in the reservoir. This variation can therefore be used to indicate to the user the fill level of the reservoir or at least the fact that a minimum level has been reached. Such information allows the user to refill the reservoir or to change a cartridge comprising such a reservoir often associated with an ink-jet print head. The capacitive arrangement is such that at least one capacitor of relatively small capacitance (a few picofarads) is constituted so that the reservoir is located or can be located at a given instant between electrodes of such a capacitor.
By way of example, the document EP 0 028 399 describes a method for detecting the minimum level of ink in a reservoir, using a resonant circuit, the capacitor of which is formed by two metal plates (electrodes) between which the ink reservoir is located. The latter therefore fills the dielectric space of such a capacitor and the quantity of ink has a direct effect on the permittivity of the said dielectric space, and therefore the value of the capacitance. The resonant circuit is calibrated so that its resonant frequency and the maximum voltage at its measurement resistance are reached when the level of ink has fallen to a predetermined value. When resonance is achieved, a signal which can be used for signalling is sent. Consequently, with such a system, the only information available is an indication of the fact that the ink level is or is not above a predetermined low threshold.
A reservoir made of insulating material is also known, comprising two compartments, respectively an upstream compartment where the consumable product (conductive ink) is in the free state and a downstream compartment where the same product is in the captive state, that is to say for example impregnating a porous body. The two compartments communicate by means of a passage while the downstream compartment filled with a porous body communicates with a flow output for consumption of the said product. As is known, the porous body makes it possible to better control the flow of the liquid to the device consuming this liquid, in particular the ink-jet print head in the case of a document printing device.
On the other hand, a larger quantity of ink can be stored, all things being equal in other respects, in the upstream compartment which lacks a porous body. The liquid contained in the upstream compartment is progressively absorbed by the porous body as the liquid flows out of the downstream compartment.
When the upstream compartment is empty, the porous body progressively contains less in liquid product, until it is completely drained.